In recent years much attention has been directed toward the development of new sources of protein for human consumption. There exists a need for protein material which can be incorporated in foods or usable as a basic proteinaceous substance for human consumption. Rapid increases in world population have made the continued dependence on traditional sources of protein highly impractical. Moreover, the supply of protein from typical sources of protein, such as animal meat and certain vegetables, is inadequate to provide balanced diets sufficient to satisfy needs of humans throughout the world. These factors coupled with the difficulties associated with providing protein from traditional sources because of drought, flooding and both animal and crop diseases gives critical significance to this situation.
One possible solution to the problem of supplying the ever increasing need for food protein is provided by processes for the bio-synthetic manufacture of protein through the growth of microorganisms on hydrocarbon or other substrates. It is known, for example, that microorganisms such as fungi, bacteria and yeast, which are grown by single-cell reproduction, contain high proportions of proteins and can be utilized directly in foods as wholecell material or can be treated to recover protein isolate. Recent efforts have shown that microorganisms, grown on hydrocarbon substrates can be successfully used in animal feeds; but as yet these microorganisms have not been commercially accepted in food preparations suitable for human consumption.
With the development of successful processes for the synthetic production of protein-containing microorganisms (sometimes referred to herein as single cell proteins), an urgent need has developed for methods of texturizing such single-cell protein materials in a manner sufficient to render them suitable for use in food products. Generally, single-cell protein is initially produced as a wet paste and then is subsequently converted into dry powder form. This dry powder, similar in appearance and feel to flour, lacks the texture and food-like sensation to the mouth necessary to make an attractive food. Moreover, when placed in water, the powdered single-cell protein rapidly reverts back to single-cell form.
Ideally, therefore, it is desirable to impart properties such as chewiness, crispness, resistance to dispersion in water and the like to such single-cell proteins in order that they may be used to full advantage as additives to and substitutes for natural foods.
Various techniques are known in the art for effecting texture formation in soy bean based protein, such techniques are not generally applicable to single-cell technology and are ineffective in such application.
The use of "texturized vegetable proteins" (hereinafter referred to as TVP) in food products, especially as meat extenders or analogs has been increasing rapidly. Many people predict that the market for TVP may reach 10% of all domestic meat consumption by the year 1985. The technology of texturizing soy protein is well established. Presently, there are mainly two types of TVP produced on the market. Namely, the expanded vegetable protein is made by a thermoplastic extrusion technique and the spun vegetable protein by a fiber spinning technique. TVP is characaterized as having structural integrity and identifiable texture. These features enable it to withstand hydration in cooking and other procedures used in preparing the food.
In order for single-cell proteins (SCP) to compete with vegetable seed proteins and to share the protein market in the future, it has to be texturized and processed for the removal of purine.
The human metabolic system produces uric acid as in the metabolism of purine. Since man does not have a uricase enzyme system, uric acid is not further broken down and is excreted with urine. Because uric acid has a very low solubility in water it will accumulate in the body in crystalline form if produced in larger quantities than the body can excrete. This may lead to the conditions known as gout and kidney stone formation. It is, therefore, recommended by many nutritionists that the purine intake in diet be kept at a low level.
Microbial cells, or single-cell protein (SCP) materials, contain from 4 to 30% or more nucleic acids according to their growth rates and the phase of growth. Usually, the higher nucleic acid contents of the microbial cells are associated with rapid growth phases. If the microbial cells are to be used as a protein source in human feeding, nutritionists recommend generally that the amount of nucleic acids contributed by SCP to diet should not exceed 2 grams per day which is equivalent to 0.36 grams of purine bases.
The calculated ribonucleic acid (RNA) contents of some conventional protein sources are given in Table I. These vary from 0 to 4%. The RNA content of SCP generally ranges from 8 to 18% for exponential growth phase cells. In SCP intended for human consumption the RNA content should preferably be reduced to about 2% on cell dry weight basis. Again, the level of 2% RNA on a cell dry weight basis is equivalent to 0.36 grams of purine bases.
TABLE I ______________________________________ RNA Content (Calculated) of Various Protein Sources FOOD % RNA ______________________________________ Milk 0 Beans 1.7 Salmon 2.4 Chicken 2.9 Beef 3.7 Pork 4.1 Liver 9.3 Anchovies 14.5 SCP 8 to 18 ______________________________________
A preferred way of utilizing SCP material is in the form of whole cells. In this form, there is a need for the development of means for removing nucleic acids from the microbial cell material. This is desirably accomplished with a minimum loss of protein materials from the cells in order to maintain the nutritional attractiveness of such SCP materials.